diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index 887e9a7bb28..c15148110a6 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -1718,8 +1718,9 @@ Return non-nil if PROJECT is not a remote project."
      (t
       (complete-with-action action buffers string pred)))))
 
-(define-completion-category 'project-buffer '(buffer)
-  "Completion category for buffers in a given project.")
+(when (fboundp 'define-completion-category) ; Introduced in Emacs 31.
+  (define-completion-category 'project-buffer '(buffer)
+    "Completion category for buffers in a given project."))
 
 (defun project--read-project-buffer ()
   (let* ((pr (project-current t))